The present invention relates to a laser diode of the surface emitting type, and more particularly to a laser diode suitable for use in technical fields such as optical communication, optical recording or regeneration, optical interconnection and optical measurement. In other words, the present invention relates to a laser diode which can be used as the light sources of an optical communication system, a laser beam printer, an optical disk apparatus, an optical gyroscope, and others.
A laser diode includes not only an edge emitting laser diode which is usually used, but also a surface emitting diode which emits a laser beam in a direction perpendicular to the upper surface of a diode chip. An example of the surface emitting laser diode is described in, for example, an article entitled "Surface Emitting GaAs/GaAlAs DFB-TJB Laser" by K. Mitsunaga et al. (The Institute of Electronics and Communication Engineers of Japan, Technical digest, OQE 86-152).
In this laser diode, an optical guide layer having a second-order diffraction grating is formed between an active layer and an upper cladding layer, and a diffracted light from the grating due to the second-order Bragg reflection is subjected to distributed feedback, to generate laser oscillation. Simultaneously with the laser oscillation, a diffracted laser beam from the grating due to the first-order Bragg reflection is propagated in upward and downward directions perpendicular to the chip of the laser diode, and is taken out through a groove which is formed in a cap layer in accordance with the shape of a light emitting area. Accordingly, the groove is provided in the form of a stripe which is extended along the axial direction of a resonant cavity.
The far-field pattern of a laser beam which is emitted in an upward direction perpendicular to the chip of the laser diode, is as follows.
(1) As to the spread of the laser beam in a direction parallel to the axial direction of the resonant cavity, light rays with the same phase are emitted from an aperture corresponding to the length of resonant cavity (for example, aperture having a length of 300 .mu.m), and hence the spread of the laser beam due to diffraction is very small. For example, the half-intensity angle of the intensity distribution of the laser beam is about 0.2.degree.. That is, the laser beam is well collimated in the direction parallel to the axial direction of the resonant cavity. PA1 (2) As to the spread of the laser beam in a direction perpendicular to the axial direction of the resonant cavity, light existing in the active layer and optical guide layer are confined in a narrow region having a width of 2 to 3 .mu.m, and hence the spread of the laser beam in the above direction due to diffraction is very large. For example, the half-intensity angle of the intensity distribution of the laser beam is about 12.degree.. In other words, the laser beam has the form of a fan when viewed in a direction parallel to the axial direction of the resonant cavity.
As mentioned above, the laser beam emitted in an upward direction perpendicular to the chip of the laser diode has the intensity distribution shown in FIG. 5A, that is, has asymmetric intensity distribution. That is, in a surface emitting laser diode having the above-mentioned structure, there arises a problem that the laser beam emitted from the surface emitting laser diode is well collimated in a direction parallel to the axial direction of the resonant cavity, but the spread of the laser beam in a direction perpendicular to the axial direction of the resonant cavity is large.